refactor: simplify data connections from many-to-many site assignment to direct site ownership

Replace SiteDataConnectionAssignment join table with a direct SiteId FK on DataConnection,
simplifying the data model, repositories, UI, CLI, and deployment service.

src/ScadaLink.Commons/Interfaces/Repositories/ICentralUiRepository.cs

@@ -10,7 +10,7 @@ public interface ICentralUiRepository
 {
     Task<IReadOnlyList<Site>> GetAllSitesAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DataConnection>> GetDataConnectionsBySiteIdAsync(int siteId, CancellationToken cancellationToken = default);
-    Task<IReadOnlyList<SiteDataConnectionAssignment>> GetAllSiteDataConnectionAssignmentsAsync(CancellationToken cancellationToken = default);
+    Task<IReadOnlyList<DataConnection>> GetAllDataConnectionsAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Template>> GetTemplateTreeAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Instance>> GetInstancesFilteredAsync(int? siteId = null, int? templateId = null, string? searchTerm = null, CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DeploymentRecord>> GetRecentDeploymentsAsync(int count, CancellationToken cancellationToken = default);